Not Applicable
Not Applicable
1. Field of the Invention
The present invention relates to electrically operated hydraulic actuators, and more particularly to such actuators of a force-feedback type which are particularly suited to operating linear actuated control valves in hydraulic systems.
2. Description of the Related Art
Construction and agricultural equipment have moveable members which are operated by hydraulic cylinder and piston combinations. The cylinder is divided into two internal chambers by the piston and alternate application of hydraulic fluid under pressure to each chamber moves the piston in opposite directions.
Application of hydraulic fluid to the cylinder historically was controlled by a manually operated valve in which the human operator moved a lever that was mechanically connected to a spool within a bore of the valve. Movement of that lever placed the spool into various positions with respect to cavities in the bore that communicate with a pump outlet, a fluid reservoir or the cylinder. Moving the spool in one direction controlled flow of pressurized hydraulic fluid from the pump to one of the cylinder chambers and allowed fluid in the other chamber to flow to the reservoir. Moving the spool in the opposite direction reversed the application and draining of fluid with respect to the cylinder chambers. By varying the amount that the spool was moved in the appropriate direction, the rate at which fluid flows into the associated cylinder chamber was varied, thus moving the piston at proportionally different speeds.
In addition, some control valves provide a float position in which both cylinder chambers are connected simultaneously via the spool to the fluid reservoir. This position allows the machine member driven by the cylinder to move freely in response to external forces. For example, a snow plow blade is allowed to float against the pavement to accommodate variations in surface contour and avoid digging into the pavement.
There is a trend with respect to construction and agricultural equipment away from manually operated hydraulic valves toward electrically controlled solenoid valves. U.S. Pat. No. 5,921,279 describes coupling a solenoid to the end of the spool to operate a control valve. Because the solenoid was capable of driving the spool in only one direction, a pair of such solenoid operated spool valves was required for each work port of the valve assembly. One of those valves controlled movement of the piston in one direction, while the other valve produced piston movement in the other direction.
It is important that the solenoid be able to accurately position the spool to meter the fluid through the valve at the desired flow rate. In an ideal valve, the position of the spool has a constant relationship to the magnitude of electric current applied to the solenoid. This ideal situation assumes that the other forces acting on the spool remain constant over the life of the control valve. In the real world, friction and other forces which affect spool movement vary as the device ages so that the same magnitude of electric current applied to the solenoid does not move the spool into the same position over time. Thus the fluid flow through the valve at a given electric current level changes during the life of the valve.
It is desirable to provide a control valve assembly that consistently locates the spool at the same position when a given magnitude of electric current is applied to the solenoid, even though when other forces acting of the spool change.
A proportional hydraulic control valve comprises a body with a bore therein, and having a work port, a supply passage, and a tank passage all of which communicate with the bore. A hydraulic motor can be connected to the work port. A pump can be connected to the supply passage and a fluid reservoir of the hydraulic system receiver fluid from the tank passage. A flow control component, such as a valve spool for example, is accommodated in the bore for reciprocal movement therein to provide a first fluid path between the work port and the supply passage and a second fluid path between the work port and the tank passage.
The proportional hydraulic control valve is operated by a force feedback actuator which has a piston that is coupled to the flow control component. The piston defines a first control chamber and a second control chamber on opposite sides of the piston in the bore. The piston has opposing ends with a depression forming a contoured surface there between that has first and second tapered sections. In the preferred embodiment, the piston has an hourglass shape.
The force feedback actuator includes a valve actuator that has a valve element which meters pressurized fluid selectively to the first and second control chambers thereby producing movement of the piston in opposite directions. That movement of the piston causes the flow control component to move into positions at which the first fluid path and the second fluid path are formed. The valve assembly including an valve actuator which produces a first force that is applied to move the valve element. A pilot pin engages the piston and the valve assembly wherein movement of the pilot pin on the first and second tapered sections of the piston transfers a second force to the valve element.
The first force from the valve actuator corresponds to a desired position for the flow control component. The second, or feedback, force indicates the actual position of the flow control component and places the valve element into a closed state when the control spool is at the desired position.
In the preferred embodiment, the linear actuator comprises first and second electrohydraulic valves. The first electrohydraulic valve includes the actuator and the valve element. The first electrohydraulic valve has a first state in which the pressurized fluid is proportionally metered to a valve outlet connected to the first control chamber, a second state in which the first control chamber is coupled to a tank passage, and a third state in which the first control chamber is isolated from both the tank passage and the source of pressurized fluid. The second electrohydraulic valve has a fourth state in which the second control chamber is coupled to the tank passage, and a fifth state in which the second control chamber is coupled to the outlet of the first electrohydraulic valve.